Even if you are not currently in a job search, you should be aware of who you would use as a reference if necessary. References are not an afterthought--they can play a key role in whether or not you get a job offer.
It's human nature to want recommendations / comments / thoughts from others before you make a big move (I bet it hasn't been that long since you looked at TripAdvisor, Yelp, or Amazon reviews). Hiring you is a very big deal for companies. They don't want to make a mistake, and they call your references more than you may have known about before now.
So...how do you maintain a good pool of references to draw on when you need them?
Make a list of people who could provide a good reference for you.
The best references are past bosses or managers, but I have seen great references come from managers you worked with but didn't report to, co-workers, direct reports, customers, and clients. The important thing is that you worked with them on something that relates to the job you want and they liked your work.
Constantly add new people who could be good references.
When you leave a job on good terms, ask your boss for their personal email and phone number so that you can stay in touch.
If someone you work with (who would be a good reference) leaves the company, ask them for their personal email and phone number so that you can stay in touch.
Consistently grow your LinkedIn connections and actively participate with others.
Keep in contact with your references list.
This is one of the reasons to keep up a routine for contacting people in your network. (See this post on Networking.) A couple of times a year, send them an email to say Hi or send them something they would be interested in. If you regularly contact the people on your list before you need them, it's not a big deal at all to call and say, "Hey, I'm interviewing for this job...would you be a reference for me?" Not only will they remember who you are, they will probably have warm fuzzy feelings for you because you have kept up the relationship.
When you do need to call on your references, coach them on what to say. Tell them what the job is, as well as key points about you or your work that would be especially helpful for your interviewer to know. Your reference will appreciate knowing ahead of time what to say that will help you out. You'll make it super easy for them, and we all appreciate that.
When you've kept up the relationship, and you prep your references, you'll know what they'll say about you (and you won't make a huge references mistake). Thinking about your references now is a fantastic investment in your future career.